The most common method of interfacing circuit cards in a computer system is to provide each circuit card with a bus connector and to plug each circuit card's bus connector into a matching socket or "slot" mounted to a backplane or motherboard. When multiple circuit cards are connected to a single backplane in this manner, the resulting arrangement is often referred to as a card cage. In higher-end computer systems of this type, circuit cards may be removed and replaced in the card cage without powering down the backplane or any of the slots except the one corresponding to the circuit card being replaced. The latter technique is known as on-line replacement or "OLR."
In order for OLR to be successful, care must be taken to prevent short circuits or other inappropriate electrical connections that may be caused by inadvertent physical contact between any of the circuit cards in the card cage. Traditional computer systems that support OLR, such as those based on the VME bus standard, have addressed this need by specifying that all circuit cards within the system must have a fixed size and shape and by providing card guides around the card cage that are consistent with the specified size and shape. Such card guides are generally effective for preventing lateral motion during the process of sliding a card into or out of the card cage, and thus aid in preventing unwanted contact between the cards during a replacement operation. In addition to using card guides and cards having a common size and shape, traditional computer systems that support OLR have further specified that the bulkhead on each circuit card should be orthogonal to the direction of extraction. When the bulkheads are oriented this way, handles may be attached to them to facilitating removal and replacement of individual cards.
Some of the more recent bus standards, however, such as the popular PCI bus standard, do not lend themselves as well to OLR operations as did previous standards. For example, PCI cards need not have a specific physical size or shape in order to comply with the defined PCI bus standard. Therefore, the shapes and sizes of PCI cards generally vary with the size and complexity of the circuitry that is implemented on them. Moreover, the bulkhead of a PCI card is disposed parallel with the direction of extraction. Therefore, it is generally not helpful to provide handles or other details on the bulkhead of the cards for removal and replacement operations. Finally, the PCI bus standard does not require that card guides be installed within the card cage. Therefore, many PCI systems designers do not provide card guides in an effort to reduce system cost.
It is an object of the present invention to facilitate removal and replacement of circuit cards in a card cage wherein the circuit cards do not have a consistent size and shape. It is a further object of the present invention to facilitate such removal and replacement of odd-sized circuit cards in a card cage while other circuit cards in the cage are not powered down.